The invention relates to an electronic driver circuit for directly modulated semiconductor lasers. The driver circuit has a first circuit for generating a constant current and a second circuit for modulating the constant current. The second circuit generates a modulating current superimposed on the constant current in dependence on a digital data signal and the superimposed current is fed to the laser.
It is known to use laser diodes for generating light in optical data transmission systems. The optical output power of a laser diode is defined here by the driver circuit that feeds to the laser diode a biasing current that is modulated as a function of the data signal which is to be transmitted.
The disadvantage of the known circuit relates to the fact that its speed is limited owing to a high output resistance of the circuit, and owing to a normally relatively high-impedance laser internal resistance (in particular when vertical cavity surface emitting lasers (VCSELs) are used). This is disadvantageous in particular at high bit rates in the gigahertz range.
It is accordingly an object of the invention to provide an electronic driver circuit for directly modulated semiconductor lasers which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which can drive directly modulated semiconductor lasers even at high data rates of up to 12 Gbit/s.
With the foregoing and other objects in view there is provided, in accordance with the invention, an electronic driver circuit for directly modulated semiconductor lasers. The driver circuit contains a first circuit for generating a constant current, and a second circuit for modulating the constant current and connected to the first circuit. The second circuit, in dependence on a digital data signal, superimposes a modulation current on the constant current resulting in a laser current to be fed to a semiconductor laser. A third circuit is provided for keeping components of the first circuit and the second circuit at a low impedance during a rising edge and a falling edge of the laser current fed to the semiconductor laser. The third circuit is connected to the first circuit and the second circuit.
According to the above, the invention provides a circuit which keeps the driver circuit at low impedance at a rising edge of the current and/or respectively at a falling edge of the current through the semiconductor laser. The driver is thus kept at low impedance during the switching on or off of the modulation current. As a result, peaking of the rising and/or falling edges of the signal or current through the semiconductor laser is achieved.
As a result of the peaking, parasitic elements of the driver circuit such as parasitic inductances and capacitances can be compensated, and an improved, optical signal shape is correspondingly made available so that the laser diode can be operated even at high data rates of up to 12 Gbit/s.
In a preferred embodiment of the invention, when the current through the semiconductor laser rises, the third circuit makes available an additional current pulse for the semiconductor laser. Such a current pulse can alternatively or additionally also be made available as a negative current pulse for the semiconductor laser when the current falls.
To this end, the third circuit preferably has a transistor whose emitter terminal is connected to the semiconductor laser via a resistor, and whose base current is determined by a base/emitter control voltage that has peaks during the rising edge of the current through the semiconductor laser.
Alternatively, there is provision for the third circuit to make available either positive or negative voltage peaks corresponding to the rising and falling edges of the current through the semiconductor laser. A constant current that is modulated in accordance with an applied base/emitter control voltage is generated at the transistor by a current mirror.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an electronic driver circuit for directly modulated semiconductor lasers, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.